Did the universe just get 15 percent larger?

Astronomers question the accuracy of a key figure used to calculate the distance of faraway galaxies.

from the April 12, 2007 edition

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But the relationship between a Cepheid's variability and its intrinsic luminosity has not been known accurately enough to serve to verify the Hubble constant. The international team worked largely with nearby Cepheids whose distance could be measured directly by geometrical means. Comparing these absolute distances with those indicated by the Cepheid's apparent brightness helped refine this relationship.

The team had to take account of myriad errors. They include minute motions of the Hubble Space Telescope that made the observations. "We've been cranking on this since 1977," Dr. Benedict says. Now this picky picky work has produced a result that Benedict says "has excited me more than any [other result] in my 35-year career."

Astronomers now have a more accurate distance-measuring tool to use wherever they can find a Cepheid.

Last August, Dr. Bonamente and colleagues reported new distance measurements to 38 galaxy clusters ranging from 1.4 billion to 9.9 billion light years away. They used radio and X-ray observations to estimate the physical size of a galaxy cluster. Geometric triangulation then gave the cluster's distance. Their check of the Hubble constant confirmed its currently accepted value.

But Kris Stanek at Ohio State University in Columbus and colleagues found that value in error. They reported last August how they measured the intrinsic brightness of a binary star system in galaxy M33. Judging the stars' distance by how dim they appear from Earth, they found it to be 3 million light years away. The estimate based on the Hubble constant's accepted value was only 2.6 million light years. The true value of the constant may be 15 percent smaller – and the universe may be 15 percent larger and older – than we thought.

Astronomers will "trust" whatever they believe is the best value for the constant. But the need to verify remains.

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